burning twice the gas!

I drive a 99 grand am with 70k. It's never had any major problems, but I understand with this many miles, it will need maintenance.

Lately, it's been running rough and burning about twice the gas it's supposed to and I've noticed it misfires especially going up hills, so I recently got my spark plugs and air filter changed (tune-up).

1- I'm having doubts that the job was done at all, so how do I get to the spark plugs on this car to make sure the spark plugs were changed?

Also, a tank of gas got me only 180 miles of city driving.

2- Any idea of what it could be?

--If this pertains: I recently got an oil change on Jan 20, so it's no where near due for one.

Buy a new air filter.
New fuel filter.
Clean MAF/IACT with MAF sensor cleaner.
New upstream HO2 sensors.

Do you have a 4 or 6 cyl? Also usually a good idea to change the spark plug wires (boots if 4 cyl) when the spark plugs are changed. Has the SES light came on? Changing the front O2 sensor(s) will net you about a 15-20% increase in gas mileage if they are getting old as they don't react as fast.